This invention is directed to an electromagnetic wave proximity sensor. In the context of this invention, the term "electromagnetic wave" includes both "microwave" and "millimeter wave," as well as other frequencies. Further, in the context of this invention, the term "microwave" refers to a frequency of less than about 30 GHz, and "millimeter wave" refers to a frequency greater than about 30 GHz. In order to keep the size of the proximity sensor as small as possible, the invention preferably operates at frequencies above about 30 GHz (millimeter wave). Hence, the following description of the invention will refer to the proximity sensor as being a "millimeter wave" proximity sensor, in order to simplify the description, but it should be realized that the invention is not limited to frequencies above about 30 GHz, or millimeter waves, and proximity sensors including the features of this invention operating at different frequencies are envisioned to be within the scope of this invention.
A proximity sensor is a non-contacting device for detecting the presence or absence of an object. Proximity sensors presently in use are of the following types:
a) Capacitive PA1 b) Inductive PA1 c) Photoelectric PA1 d) Ultrasonic PA1 e) Microwave Doppler (Motion) PA1 1) A means for short range proximity sensing in harsh industrial or natural environments such as dust (e.g. grainery, mines), smoke, fog, vapor (e.g. degreaser, steam, hydrocarbon exhaust), hydrocarbon fluids (oil, grease), cleaning liquids or sprays (hydrocarbon or organic based, e.g. carbon tetrachloride, benzene, xylene, trichlorethylene), dirt, flames (fire fighting, furnaces). PA1 2) A means to provide uniform detected output of an object arbitrarily located in the spatial range of the millimeter wave proximity sensor herein disclosed. PA1 3) A means to provide a low cost transmitter source for the disclosed sensor. It is based on a new Gunn diode oscillator circuit with the following novel features: PA1 a transmitter for transmitting radiated energy to a target, the transmitter including a Gunn oscillator circuit, the Gunn oscillator circuit including a Gunn oscillator and a Gunn driver coupled to the Gunn oscillator, the Gunn oscillator, driven by the Gunn driver, generating a pulse modulated signal which is radiated by the transmitter to the target; PA1 a receiver, the receiver including first and second waveguides and first and second detectors respectively coupled to the first and second waveguides, the first and second waveguides receiving radiated energy reflected from the target and providing the radiated energy to the first and second detectors, respectively, each of the first and second detectors generating an output signal which corresponds to the radiated energy received by the first and second waveguides, the receiver further including a summer, the summer being coupled to the first and second detectors and being responsive to the output signals of the first and second detectors, the summer generating an output signal corresponding to the sum of the output signals of the first and second detectors; and PA1 an analog processor/driver circuit, the analog processor/driver circuit being responsive to the output signal of the summer, the processor/driver circuit including a pulse modulator generating a pulsed signal and providing the pulsed signal to the Gunn driver, an amplifier receiving the output signal of the summer and generating an amplified signal in response thereto, a synchronized demodulator signal conditioner being responsive to the amplified signal and generating an output signal in response thereto, and an output stage responsive to the output signal from the synchronized demodulator signal conditioner and generating an output signal in response thereto.
It is the purpose of the new millimeter wave (frequency greater than 30 GHz) active proximity sensor, herein disclosed, to provide short range (from face of sensor to a few feet) sensing capability in a harsh environment, said capability not being presently available with known proximity sensors. The new features of the millimeter wave sensor are as follows:
An inductive type proximity sensor may be used in a harsh environment but it lacks in sensor range (0.6 inches typical for a 30 mm sensor diameter). The inductive sensor is also known to detect only metallic objects and that an increase in target size will not produce an increase in sensing range. The disclosed millimeter wave proximity sensor of the present invention provides at least a 6 times increase in detection range, relative to an inductive sensor, and its sensing range increases with target size, as optimally & theoretically shown by the graph in FIG. 1. In addition, the millimeter wave proximity sensor can detect a variety of materials, including metals, dielectrics and various liquids.
Capacitive sensors have a greater range (1.6 inches max.) than an inductive sensor, as disclosed in The Basics of Inductive and Capacitive Proximity Presence Sensing, Eaton Corp. Milwaukee, Wis., but are unsuitable for harsh environments due to the very significant and unacceptable effect that a harsh environment has on the operating mechanism of a capacitive sensor. Photoelectric and ultrasonic sensors rely on a non-absorbing and non-dispersive medium between their respective transmitter and the object to be detected. Most harsh environments are absorptive and dispersive, and hence are unsuitable for an ultrasonic or photoelectric proximity sensor. By contrast, the harsh environments previously exemplified are essentially transparent to millimeter wave electromagnetic radiation and the millimeter wave proximity sensor of the present invention can function effectively in said environments to detect the presence or absence of an object.
Doppler based microwave or millimeter wave sensors are intended for far field detection rather than near field (short range) detection of an object in motion and not for the detection of a stationary object. The Doppler type sensor is also not suitable for slow motion sensing due to detection sensitivity limitations incurred by the return signal being in a frequency region very close to the transmitter frequency. The signal frequency offset from the carrier decreases as the speed of the object in motion decreases.
The sensor operational range is in what is known as the near field region of the transmitter output. There is a known spatial non-uniformity of the electromagnetic field in this near field region as the output of the transmitter transitions from the transmitter output waveguide port to free space. The radiated wave is non-planar in the near field region. (It transitions to a plane wave in the far field.) Due to the non-uniform spatial field distribution, it is known that sensor detection nulls from an object will be experienced at some locations in the spatial range of the sensor. A unique feature of the disclosed proximity sensor is a means to alleviate these detection nulls.
A means for self frequency compensation with temperature. PA2 Elimination of the need for an output isolator that is conventionally required to prevent excessive frequency change, frequency discontinuity or performance drop out with changes in output load. PA2 Elimination of the need for a matching circuit between the output waveguide and the oscillator circuit. A matching circuit is conventionally used to ensure maximum output power to the load. PA2 Use of the Gunn oscillator output waveguide directly as a slot type antenna, thereby eliminating the need for an external antenna. PA2 Use of a Gunn oscillator waveguide circuit that has been reduced to an elemental form. The discrete circuit elements are a Gunn diode and a chip capacitor.
One of the objects of this invention is to provide a low power, low cost, millimeter wave (MMW) proximity sensor that provides over a 6:1 increase in minimum detection range (55 mm minimum) compared to standard 18 mm inductive proximity sensors. The design preferably includes a low cost MMW Gunn oscillator, MMW detector and analog processing/driver circuitry. The sensor is preferably designed for 3-wire operation and will fit in a standard 18 mm tube.
An initial cost analysis of the sensor assembly has been completed and results indicate that a manufactured cost of less than $100, in quantities of greater than 10,000, can be achieved.
The sensor provides approximately a 6:1 improvement in minimum detection range compared to 18 mm inductive sensors. For an 18 mm target, a standard 18 mm inductive sensor would provide 9 mm of sensing range as compared to the MMW sensor which will provide 55 mm of sensing range. An important difference between the MMW proximity sensor and conventional inductive sensor technology is that the sensing range of the present invention is a function of target size. Therefore, the sensing range of the proximity sensor of the present invention will be significantly greater for larger targets (i.e. &gt;30:1 theoretical improvement for a 100 mm target).
A perspective view of the low cost, low power, MMW Gunn oscillator of the present invention is presented in FIG. 4. A simple monopole element is used as both the tuning circuit and interface between the Gunn diode and the output of the sensor. This greatly simplifies the circuit topology and therefore the assembly cost. This simple, robust circuit design results in a low manufactured cost and eliminates any need for circuit alignment. Two breadboarded oscillators both achieved +10 dBm output power at 35 GHz, thus meeting a 0 dBm nominal output power design goal. Excellent repeatability was observed from breadboard to breadboard.
A perspective view of the low cost MMW detector circuit of the present invention is shown in FIG. 5. The circuit was realized using a simple printed circuit and a low cost, commercially available, GaAs Schottky diode which was designed for pick and place type assembly. This resulted in an extremely simple, highly produceable, low cost circuit. Two breadboarded MMW detector circuits provided good sensitivity and very flat performance over the 30 to 37 GHz frequency range, thereby permitting a decrease in the oscillator frequency accuracy. Excellent repeatability was observed from breadboard to breadboard.
The MMW proximity sensor and a photoelectric sensor product manufactured by Eaton Corporation are similar from a functional point of view. Due to this similarity, the Eaton photoelectric ASIC was able to be used in the analog processing/driver circuitry of the present invention. This greatly reduces the circuit complexity and therefore reduces the manufactured cost. A breadboard of the analog processor/driver circuits was built and evaluated. Initial results looked excellent and the board met all design goals. A top plan view of the analog processor/driver board is presented in FIG. 6.
In accordance with one form of the invention, an electromagnetic wave, reflective type, active proximity sensor comprises:
It is to be appreciated, as will be described in greater detail later, that the two waveguide/two detector embodiment described above may be implemented in order to eliminate detection nulls. Further, while the particular embodiments of the receiver of the present invention, discussed herein, refer to a direct detection technique of detecting the reflected signal, it is to be understood that the receiver may be in the form of a homodyne mixer whereby the received signal is actually mixed with a delayed version of itself. Such a configuration may provide improved detector sensitivity.
In accordance with another form of the present invention, an electromagnetic wave, reflective type, active proximity sensor includes a transmitter, a receiver and means for substantially eliminating a leakage signal (i.e., portion of transmitted output signal that leaks into the receiver) which is received by the receiver after being radiated by the transmitter. In one embodiment of the leakage signal elimination means, a beam splitter with a pre-determined thickness is positioned between the sensor and the target at a pre-determined separation distance. Accordingly, the output signal radiated by the transmitter strikes the beam splitter, and in response, the beam splitter generates a reflected signal portion and a transmitted signal portion. The transmitted signal portion continues on toward the target where it is reflected back to the sensor and received by the receiver. However, the reflected signal portion, which is approximately 180 degrees out of phase with the leakage signal, reflects back to the receiver from the beam splitter and substantially cancels the leakage signal, resulting in improved detector sensitivity. It is to be appreciated that the function of the beam splitter, in one embodiment of the present invention, may be provided by the dielectric cover shown in FIG. 3b of the drawings.
In another embodiment of the leakage signal elimination means, the transmitter and receiver each include a circular waveguide and a polarization circuit. The circular waveguide and polarization circuit of the transmitter serve to circularly polarize the output signal generated by the oscillator circuit in a first circular direction (i.e., either clockwise or counter-clockwise). The circularly polarized output signal is then radiated toward the target where it strikes the target and reflects back in the form of a return signal rotating in a second circular direction opposite to the first circular direction. Next, the return signal is received by the circular waveguide of the receiver and circularly de-polarized (i.e., returned to a linear polarization) by the receiver polarization circuit. The circularly de-polarized signal is then processed by the detector. In this manner, the receiver is responsive only to signals polarized in the second circular direction and, thus, the leakage signal radiated by the transmitter, polarized in the first circular direction, is not de-polarized by the receiver and, therefore, not processed by the detector.
In accordance with yet another form of the present invention, an electromagnetic, reflective type, active proximity sensor includes a transmitter having a voltage controlled oscillator circuit which generates an output signal which is modulated such that the frequency of the output signal varies over time between a range of frequencies. In this manner, return signals which correspond to the radiated energy reflected from the target and respectively exhibit one of the frequencies corresponding to the range of frequencies associated with the frequency varying, output signal radiated by the transmitter, are received by the receiver. The detector of the receiver averages the return signals and, in response, generates an output signal corresponding to the return signals. It is to be appreciated that the output signal generated by the voltage controlled oscillator circuit may discretely vary between at least two frequencies or, in another embodiment, sinusoidally vary between at least two frequencies.
It is to be appreciated that another object of the present invention is to provide an oscillator circuit which oscillates, thereby generating an output signal, without the need for a separate resonator. In other words, the present invention provides a self-resonating oscillator circuit in the form of a novel Gunn oscillator circuit. The discrete circuit elements, which will be described in greater detail later, preferably include a Gunn diode and a chip capacitor coupled by an inductive line having a length of approximately one quarter wavelength. It is to be appreciated that, in a preferred embodiment, the chip capacitor and the inductive line constitute a low pass filter for applying bias voltage to the Gunn diode. A post is coupled to the Gunn diode in order to provide the output signal, generated by the circuit, to a waveguide to which the circuit may be coupled. It is also to be appreciated that the oscillator circuit may be modified such that the Gunn diode may be replaced with other active elements such as an Impatt diode, an FET or an HEMT device, while still achieving the advantages of the present invention discussed herein.
A comprehensive description of the preferred forms of the proximity sensor are presented in the following section, with reference to the drawings.